1. Field of the Invention
The present invention relates to a power supply circuit of portable devices and, particularly, to a circuit of selecting and detecting a power supply.
2. Description of Related Art
Recent information technology devices are reduced in size, and mobile computers, digital cameras, and storage devices with high portability are developed to thereby enable information interchange with interconnection. Such portable devices are small sized to ensure portability, and therefore power is supplied through a power-suppliable interface connector such as an AC adapter or USB (Universal Serial Bus).
Such a power supply system is often designed to, for example, place high priority on a power supply, if any, from a power supply source dedicated to providing a power supply only such as an AC adapter in order to minimize the power consumption of a battery of a host computer or the like which is connected through an interface connector. For instance, a circuit to detect and select a power supply is placed between a portable device and a power supply source. Power supply selection/detection circuits according to related arts are described hereinafter.
FIG. 5 is a block diagram of a power supply selection/detection circuit according to a relate art. In the power supply selection/detection circuit 500, a voltage supplied from a power supply 511 or a power supply 521 is applied to a load 501 through a main power supply 502. The power supply selection/detection circuit 500 includes a power supply detector 510 disposed between the power supply 511 and the main power supply 502, and a power supply detector 520 disposed between the power supply 521 and the main power supply 502. The power supply detector 510 includes a Schottky diode 512, a voltage comparator 513, and two resistors 514 and 515. The power supply detector 520 includes a Schottky diode 522, a voltage comparator 523, and two resistors 524 and 525.
The anode of the Schottky diode 512 is connected to the power supply 511, and the cathode of the Schottky diode 512 is connected to the main power supply 502. The voltage comparator 513 detects the voltage of the power supply 511 to determine the presence or absence of the power supply 511. The voltage comparator 513 receives a reference power supply voltage Va1 and a voltage Va2 at a node between the resistors 514 and 515 that divide by resistance the power supply 511. The anode of the Schottky diode 522 is connected to the power supply 521, and the cathode of the Schottky diode 522 is connected to the main power supply 502. The voltage comparator 523 detects if power is supplied through the power supply 521. One input terminal of the voltage comparator 523 receives a reference power supply voltage Vb1, and the other input terminal of the voltage comparator 523 receives a voltage Vb2 at a node between the resistors 524 and 525 that divide by resistance the power supply 521.
In this example, the power supply 511 is described as an AC adapter and the power supply 521 as an USB connector, for example. Thus, when power is supplied from an AC adapter, power supply from an interface is shut off, for instance. The operation of the power supply selection/detection circuit 500 when the voltage supplied from the AC adapter is higher than the voltage supplied from the USB connector, which is when the power supply 511 is higher than the power supply 521, is described hereinafter. The voltage comparator 513 compares the reference voltage Va1 with the voltage Va2 at the node between the resistors 514 and 515 to detect that the power supply 511 is supplied. On the other hand, the voltage comparator 523 compares the reference voltage Vb1 with the voltage Vb2 at the node between the resistors 524 and 525 to detect if the power supply 521 is supplied. If the power supply 521 is lower than the power supply 511, the voltage from the power supply 511 is supplied preferentially.
The operation of the power supply selection/detection circuit 500 when only the power supply voltage from the AC adapter is connected, which is when only the power supply 511 is connected, is described hereinafter. The voltage comparator 513 compares the reference voltage Va1 with the voltage Va2 at the node between the resistors 514 and 515 to detect that the power supply 511 is being supplied. On the other hand, reverse leakage current Ir from the main power supply 502 flows to the Schottky diode 522. Although diodes ideally transmit current in one direction only, backward current flows slightly in practice. Accordingly, the voltage Vb2 occurs at the node between the resistors 524 and 525. If the voltage Vb2 at the node is not lower than the reference voltage Vb1, the voltage comparator 523 falsely detects that power is being supplied through the power supply 521. It is therefore necessary to set low values to the resistors 524 and 525.
Another example of a power supply selection/detection circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2000-284865. FIG. 6 is a block diagram of a power supply selection/detection circuit disclosed therein. The power supply selection/detection circuit illustrated in FIG. 6 includes a power supply line 1 to supply power from an interface such as USB and a power supply line 2 to supply power from an AC adapter. In this circuit also, power supply from the interface is shut off when power is supplied from the AC adapter. Thus, if the voltage of the power supply line 2 increases, the current flows into the ground through a Zener diode 10. Accordingly, the base voltage of a transistor 17 increases due to the IR drop across a resistor 12 to turn on the transistor 17. The base voltage of a transistor 18 thereby decreases to turn off the transistor 18, which then turns off a transistor 20. As a result, the power supply from the interface is shut off.
However, the inventor of the present invention has found that the above-described related arts have the following problems. In the power supply selection/detection circuit 500 illustrated in FIG. 5, if the voltage supplied from the power supply 511 is higher than the voltage supplied from the power supply 521, current flows to the resistors 524 and 525 which are connected to the voltage comparator 523. This raises the problem that current consumption occurs in the power supply detector 520 that is related to the power supply 521 which is not connected with the main power supply 502. Although the consumption current can be reduced by increasing the resistance of the resistors 524 and 525, this causes another problem. For example, if the voltage is supplied from the power supply 511 only, the reverse leakage current Ir from the main power supply 502 flows to the Schottky diode 522, and the voltage Vb2 occurs at the node between the resistors 524 and 525. Thus, if the voltage Vb2 at the node is not lower than the reference voltage Vb1, the voltage comparator 523 can falsely detect that power is being supplied through the power supply 521.
To avoid this, if the resistance of the resistor 525 is Rb2, the reverse leakage current of the Schottky diode is Ir, the reference voltage of the voltage comparator 523 is Vb1, it is necessary to set the resistance of the resistor 525 to satisfy the expression: Rb2*Ir<Vb1, and therefore the resistance of the resistor 525 cannot be set high enough. This raises the problem that the current consumed in the power supply detector 520 cannot be reduced when power is supplied from the power supply 511 to the main power supply 502. This not only increases the power consumption in the circuit in the standby mode but also fails to satisfy the current limit Imax of an USB connector in suspend mode if the power supply 521 is a USB connector.
The similar problem occurs in the configuration of the power supply selection/detection circuit illustrated in FIG. 6. Because current keeps flowing through the resistors 11 and 12 while the AC adapter is connected, power consumption occurs in a voltage detector 5. Although the consumption current can be reduced by increasing the resistance of the resistors 11 and 12, this causes another problem. Specifically, diodes 8 and 9 are placed respectively on the power supply lines 1 and 2 to prevent current backflow. Although the diodes ideally transmit current in one direction only, backward current flows slightly in practice. For example, if power is supplied through the power supply line 1, the backward current occurs in the diode 9 and flows into the ground through the resistors 11 and 12. Then, even though no power is supplied through the power supply line 2, the base voltage of the transistor 17 increases due to the IR drop across the resistor 12. This turns off the transistor 18 and further turns off the transistor 20; as a result, the power supply from the power supply line 2 is shut off. To avoid this, the resistance of the resistor 12 cannot be set high enough. Consequently, the current consumed in the power supply selection/detection circuit cannot be reduced.